Electromagnetic (EM) pumps utilizing the electrical conductivity of liquid metals such as sodium or a sodium-potassium alloy are known. The absence of movable parts in such pumps provides simplicity and reliability over conventional mechanical pumps. However, the application of electromagnetic pumps to the main coolant circulation systems of liquid metal reactors has been limited by safety considerations, by the inherent bulkiness of conventional design approaches, and by difficulties in designing electrical insulation for high temperature and high voltage applications.
The present annular linear induction pump can replace either the primary or secondary mechanical pump typically used in liquid metal cooled nuclear reactors. EM pumps for liquid metal circulation systems have been limited to small size applications (less than 6,000 gpm). Large pumps have required active cooling systems for the electrical conductors in order to keep the copper and its electrical insulation within allowable operating limits. In addition, the availability of electrical insulation capable of functioning at the high temperatures (approximately 500.degree. C.) and high voltage (approximately 6,000 volts) typically experienced in conventional reactors has been a design problem requiring research and development. The introduction of a pump cooling system complicates the reactor design and reduces system efficiency. Additionally, the pump coolant system complicates the reactor installation. See, "Electromagnetic Pumps For Large Pool-Concept LMFBR", Research Institute Report No. NP-1265, (Palo Alto 1979), U.S. Pat. Nos. 4,527,955 and 4,505,644.
A serious drawback of prior art electromagnetic pumps is their inability to sustain coolant flow upon loss of electrical power. In the event of a power outage or plant shutdown, prior art electromagnetic pumps cease functioning. Mechanical pumps can be designed with a flywheel as part of the motor assembly to provide a slow coast-down in flow rate. Such a safety feature can assure that the nuclear reactor fuel assemblies are maintained within an acceptable temperature range. Conventional electromagnetic pumps require a motor generator set to assure a slow coat-down and costly diesel or gas turbine generator systems to provide enough alternating current to maintain coolant flow at a rate sufficient to remove decay heat from the reactor's fuel elements.
The liquid metal circulation system of the present invention, including a uniquely configured electromagnetic pump, optionally connected to one or a series of fluid flow couplers, can replace the conventional primary and secondary coolant pumps of a nuclear reactor. The flow coupled is a specialized direct current electromagnetic generator and pump connected together in a common magnetic field. The direct current linking the primary and intermediate flows is generated locally, enabling the use of higher current/lower voltage than would be possible with a traditional direct current pump. See, for example, U.S. Pat. No. 4,469,471. Despite these advantages, the flow couplers of the prior art still retain several drawbacks. Though smaller than standard direct current electromagnetic pumps, prior art flow couplers utilize electromagnets to magnetically interconnect the liquid metal flows through the opposing parallel flow paths. The inherent bulkiness of this design feature complicates installation. The preferred flow couplers contemplated herein utilize permanent magnets instead of electromagnets and thus eliminate the need for a constant power source.